1. Technical Field
The present invention relates to an imaging apparatus, an imaging method, and a focus control apparatus.
2. Related Art
Many imaging apparatus of related art, such as a monitoring camera and a DVD (digital versatile disc) camera, have a built-in autofocus function that allows automatic focus adjustment. An example of a focusing method in an autofocus function of this type is a contrast-based focus adjustment method assuming that the amplitude of a contrast signal provided from captured video images is maximized when the images are brought into focus.
In an imaging apparatus, when, a focusing lens is moved along the optical axis thereof, captured video images are defocused or brought into focus and the amplitude of the contrast signal changes accordingly. Therefore, in a fundamental contrast-based method, the focusing lens is moved along the optical axis thereof, and a direction in which images are brought into focus is detected based on the magnitude of the amplitude of the contrast signal before and after the movement, followed by movement of the focusing lens in the detected direction to achieve an in-focus state.
In autofocus control, in which a focusing lens in a lens unit is moved along the optical axis thereof, performing unnecessary autofocus control excessively wears the lens unit and hence lowers the durability of the imaging apparatus.
For example, there is, for example, a situation in which a subject suddenly appears in front of a camera apparatus that has achieved an in-focus state of an image of another subject and the subject in front of the camera apparatus then instantly disappears from the screen of the camera apparatus. Also in this case, the camera apparatus determines that the another subject has changed and reactivates autofocus control, which means that unnecessary autofocus control is performed and the durability of the imaging apparatus is lowered. Further, since unnecessary focusing control is performed frequently, a risk of incorrect positioning of the focusing lens in positions shifted from a correct in-focus position increases. Moreover, when the focus control is continuously performed in response to a subject that suddenly appears in the screen of the camera apparatus, the focusing lens travels back and forth between the in-focus position and an out-of-focus position, resulting in continuous display of disordered video images. It is therefore necessary to prevent the durability of the imaging apparatus from being degraded by unnecessary autofocus reactivation and provide sharp video images.
An example of a background technology in the present technical field is JP-A-2006-208818. JP-A-2006-208818 describes “When a video camcorder that performs contrast-based autofocus control detects that an evaluation value used to detect an in-focus/out-of-focus state changes from a value representing an in-focus state by at least a threshold that serves as a guide for autofocus reactivation, the video camcorder is first put on standby until a standby period TW elapses. Ina case where the standby period TW elapses but the detected evaluation value remains unchanged without returning to a value smaller than the threshold, the autofocus control is reactivated. The video camcorder can thus be so operated that it does not respond when the evaluation value instantly changes by an amount greater than or equal to the threshold” (see Abstract). JP-A-2006-208818 therefore discloses a technology for evaluating whether such an out-of-focus state has occurred and activating autofocus control accordingly.
The method described in JP-A-2006-208818 uses the configuration in which the apparatus is put on standby for a fixed period and activates autofocus control only when the evaluation value changes by a fixed amount or smaller. However, even in a case where it is necessary to immediately reactivate autofocus control, it may also take time to perform the reactivation evaluation because the evaluation is performed after the apparatus is put on standby for the fixed period.